Amplicons were sequenced by BMR Genomics (Padova, Italy). Acknowledgements This work was supported by Progetti di Ricerca di Ateneo 2006 from the University of Padova (prot. CPDA063434)

and Ricerca Scientifica fondi quota EX 60% 2007-2009 (prot. 60A06-9994/07, 921/08 and 5430/09) to L.N. We are grateful to M. Brini (Padova, Italy) for kindly providing the apoaequorin cassette and for helpful discussion on Ca2+ measurement experiments, and to D. Sanders (York, UK) for critical reading of the manuscript and insightful comments. We also thank J. Stougaard (Aarhus, Denmark), S. Varotto (Padova, Italy) and Vergerio Mangimi S.R.L. (Padova, Italy) for the kind gift of L. japonicus, soybean and V. sativa seeds, respectively. AZD0530 research buy Electronic supplementary material Additional file 1: Map of the apoaequorin-expressing plasmid AZD2281 price pAEQ80. Abbreviations: P, IPTG-inducible synthetic promoter (Psyn); HA1-AEQ, cloned apoaequorin cDNA with hemoagglutinin epitope; KmR, kanamycin resistance gene; lacIq, constitutive lac repressor gene. Relevant restriction endonuclease sites are also shown. (TIFF 182 KB) Additional file

2: Validation of the aequorin-expressing M. loti experimental system. A, Analysis of aequorin expression in M. loti based on an in vitro reconstitution assay. Data are the means ± SEM of three experiments. B, Effect of pAEQ80 plasmid and expressed recombinant apoaequorin on M. loti cell growth. Data are the means of two independent experiments. C, Nodulated root of L. japonicus

4 weeks after inoculation with the recombinant M. loti strain. Bar = 2 mm. D, DAPI staining of M. loti cells USDA 3147T pAEQ80 squeezed from a young nodule. Bar = 10 μm. E, Monitoring of intracellular Ca2+ concentration ([Ca2+]i) Clomifene in resting M. loti cells grown to mid-exponential phase. (TIFF 5 MB) References 1. Oldroyd GED, Downie JA: Coordinating nodule morphogenesis with rhizobial infection in legumes. Annu Rev Plant Biol 2008, 59:519–546.CrossRefPubMed 2. Garg N, Geetanjali : Symbiotic nitrogen fixation in legume nodules: process and signaling. A review. Agron Sustain Dev 2007, 27:59–68.CrossRef 3. Cooper JE: Early interactions between legumes and rhizobia: disclosing complexity in a molecular dialogue. J Appl Microbiol 2007, 103:1355–1365.CrossRefPubMed 4. Norris V, Grant S, Freestone P, Canvin J, Sheikh FN, Toth I, Trinei M, Modha K, Norman RI: Calcium signalling in bacteria. J Bacteriol 1996, 178:3677–3682.PubMed 5. Dominguez DC: Calcium signalling in bacteria. Mol Microbiol 2004, 54:291–297.CrossRefPubMed 6.